1. Field of the Invention
This invention relates to a multiplex transmission apparatus connected to other multiplex transmission apparatuses by a common transmission line to construct a multiplex transmission system for carrying out data communications among the multiplex transmission apparatuses.
2. Description of the Related Art
Multiplex transmission apparatuses of this type include the one shown in FIG. 12 (RELATED ART). As shown in FIG. 12, a multiplex transmission apparatus 10 comprises a bus interface circuit 11, a communication control circuit 12, a receiving buffer memory (hereinafter merely referred to as "receiving buffer") 13, an output port 14 including a latch circuit etc., an output circuit 15, loads 16 such as motors, lamps and the like, loads 17 such as various switches, an input circuit 18, an input port 19, a transmitting buffer memory (hereinafter merely referred to as "transmitting buffer") 20, a power circuit 22, a low-voltage detecting circuit 23, and a timer IC 24 including a watchdog timer 25 and a monostable multivibrator 26. The communication control circuit 12, the output port 14 and the input port 19 are constituted by a single communication IC 21.
The multiplex transmission apparatus 10 is connected to a transmission line MB of a multiplex transmission system through the bus interface circuit 11. Frames of data are transferred via the transmission line (bus) MB among a plurality of multiplex transmission apparatuses 10, 30, etc.
The multiplex transmission apparatus 10 is supplied with electric power from an external battery, and a power supply voltage of 5V is applied to the individual internal circuits by the power circuit 22. The low-voltage detecting circuit 23 is connected to the power circuit 22 and the communication control circuit 12 of the communication IC 21. On detecting a drop of the power supply voltage to a predetermined voltage value at which the multiplex transmission apparatus cannot operate normally, the low-voltage detecting circuit 23 provides an output so as to reset the entire communication IC 21, thereby preventing erroneous operation of the multiplex communication apparatus.
As a measure of the multiplex transmission apparatus 10 against abnormality of the transmission system, the watchdog timer 25 of the timer IC 24 is supplied with data from at least one specific output port, for example, data which periodically and alternately turns to high (hereinafter "H") and low (hereinafter "L") levels, among a plurality of data items output from the output port 14.
The watchdog timer 25 monitors the repetition periods of the "H" and "L" levels input thereto by their trailing edges from "H" to "L" level, and in the event of an improper period being detected, the timer 25 outputs a reset signal. Thus, the multiplex transmission apparatus 10 is constructed so that the transmission system may not operate erroneously in case of abnormality.
When the signal monitored is abnormal, the watchdog timer 25 outputs the reset signal which alternately turns to "H" and "L" levels at predetermined intervals, but this pulse width is varied by the monostable multivibrator 26 incorporated in the timer IC 24.
The monostable multivibrator 26 sets a circuit constant such that the "L"-level pulse lasts for a predetermined length from the trailing edge of the reset signal from "H" to "L" level. Accordingly, in the event abnormality occurs, the timer IC 24 outputs a continuous "L"-level signal with a fixed length. This "L"-level signal is supplied to the communication control circuit 12 via an input terminal 21a of the communication IC 21.
On detecting the "L" state of the output from the timer IC 24, the communication control circuit 12 judges that the transmission system is in an abnormal state. Then, the communication control circuit 12 immediately brings part or all of the output port 14 to a predetermined specific state, for example, high-impedance state, to thereby keep the loads connected to the output port 14 safe.
When the transmission system is recovered to normal state, the output of the timer IC 24, which has been in the "L" state after the occurrence of abnormality, must be changed to "H" level so that the output port 14, which is in the specific state, can output data normally. Namely, the specific output port must be set again so as to output data which periodically and alternately turns to "H" and "L" levels.
The specific output port is not brought to high-impedance state even when the input terminal of the communication IC 21 is in the "L" state, because data should be output as soon as frame reception becomes available.
In multiplex transmission apparatuses, a power supply voltage threshold (hereinafter referred to as "reset voltage") used for avoiding erroneous operation is set to a voltage at which even part of the circuitry in the apparatus may possibly fail to operate normally. Where the multiplex transmission apparatus is used in a multiplex transmission system for an automotive electrical system, for example, a large quantity of electric current flows during rotation of the starter motor at the startup of the engine (hereinafter referred to as "cranking") or in similar conditions, and accordingly, the battery voltage lowers. As a result, the power supply voltage may drop to the reset voltage.
In such cases, in the multiplex transmission apparatus, an "L"-level signal is supplied from the timer IC 24 to the input terminal 21a of the communication IC 21; therefore, the communication control circuit 12 is reset and thus becomes unable to receive data. Since in this case the output of the specific output port does not alternately turn to "H" and "L" levels, the watchdog timer 25 outputs the reset signal. Consequently, the timer IC 24 outputs an "L"-level signal, and the state of the output port changes to high-impedance state which is the predetermined specific state.
In a system wherein headlamps as the loads 16, for example, are driven by the multiplex transmission apparatus 10, the output port is pulled up so that no danger may be incurred by potential operation of the loads in case of abnormality. Further, the system may be constructed in such a manner that when the output port changes to the high-impedance state, the headlamps are lighted. In this case, however, the headlamps are uselessly lighted every time the engine is started.
Furthermore, in the case where the multiplex transmission apparatus is used in a multiplex transmission system for an automotive electrical system, certain functions such as the door locking/unlocking function must be available even while the ignition switch is off. In some systems, therefore, while the ignition switch is off, a power economy mode (hereinafter referred to as "sleep mode") is executed in which the operation is stopped and only changes in the input switch signals are monitored, to thereby reduce the load on the battery and thus the electric current consumed. Even in such systems, since the operation is stopped while the ignition switch is off, application of the signal (alternate "H"-and-"L" signal) to the watchdog timer also ceases. Consequently, the timer IC outputs an "L"-level signal, which brings the output port to the specific state.